You have the need to know, but his/her lips are sealed or walks away, ignores or bypasses your questions, puts it back on you, denies or does other numbers on you to keep you away from knowing what really happened. There are predictable reasons why this happens. Here are some of them:

1. Your partner/spouse is involved in an "I don't want to say no" affair. Your partner probably is a person wrapped up in his own ego, personal needs, and life style. He can rationalize the behavior and actually come to the conclusion that he is entitled to get his needs met this way. After all, he is such a magnanimous person! Bottom line: your needs and concerns really don't matter! He doesn't want to talk about them and sees absolutely no reason to get "bogged down" in what is important to you.
2. Your spouse/partner is involved in an "I can't say no" affair. Your partner finds it painfully difficult to respond to your need to know out of shame and guilt. She sees, at one level, her actions becoming more and more destructive and degrading and believes, again at an unspoken level usually, that she has less and less control over her actions. Guilt and shame follow the infidelity with self-promises to break off the behavior. However, these promises are usually broken. She is ashamed for you to know this struggle.
3. Your partner/spouse is involved in a "revenge" affair. He wants you to squirm. This may not be front and center in his consciousness, but just below the surface is some resentment and anger, for whatever reason, aimed in your direction. He thinks: "Hmmmm this is payback time. Good. Now she knows what it's like to be on the receiving end. I'll continue this for a while...and secretly enjoy her torment. I won't give her the satisfaction of responding in a caring way to her needs."
4. Your partner/spouse is involved in an affair with the intent upon proving her desirability. In some cases where there is a history of sexual abuse as a child, or rape as an adult, your partner may compartmentalize the "affair" to the degree that she might not consciously remember the details or events of her infidelity. The infidelity may serve in a pre-conscious fashion as an attempt to amend for the painful sexual history. She may NOT indeed remember what you ask for.
5. Your partner/spouse is involved in affairs such as: 'I fell out of love" or "My Marriage Made Me Do It" or "I Want to be Close to Someone...but can't stand intimacy." Often, the infidelity in these cases represents the need to deal with dependency issues. By that I mean, your partner may define himself in terms of how others respond to him rather than his inner values, standards, purpose, etc. This person's life is wrapped around others. And his life is still wrapped around you. You want to know. He doesn't tell you... for fear of "hurting you" or becoming embroiled in pain or conflict from which he cannot seemingly extricate. What you (or others) think, feel and how you respond are TOO important to him.

As you see, the reasons for not getting the information you need for your own sense of validation and acknowledgement are varied and fairly complex. Allow yourself to stand back and examine the themes and patterns you encounter. With the power of this knowledge you gain the freedom to use different strategies and tactics to work toward resolution.